Image forming apparatuses (below called liquid-ejecting image forming apparatuses) are known which eject liquid droplets onto a sheet material such as a sheet of paper to form an image and produce printed matter. The liquid ejecting image forming apparatuses may generally be divided into a serial-type image forming apparatus and a line-head type image forming apparatus. In the serial-type image forming apparatus, a recording head thereof moves in both main scanning directions perpendicular to a direction of sheet conveying while the sheet conveying is repeated to form an image over the sheet of paper to produce printed matter. In the line head-type image forming apparatus with nozzles being aligned in a length which is almost the same length as a maximum width of the sheet of paper, when timing arrives at which the sheet of paper is conveyed and the liquid droplets are ejected, nozzles within the line head eject the liquid droplets to form the image.
However, it is known that, in the serial-type image forming apparatus, when one ruled line is printed in both directions of an outward path and a return path, an offset of the ruled line is likely to occur between the outward path and the return path. Moreover, it is known that, in the line head-type image forming apparatus, parallel lines are likely to appear in the sheet-conveying direction when there is a nozzle whose position of impacting is constantly offset due to a mounting error, finishing accuracy of the nozzle, etc.
Therefore, in the liquid-ejecting image forming apparatus, it is often the case that a test pattern for self-adjustment to adjust the position of impacting the liquid droplets is printed on the sheet material, the test pattern is optically read, and an ejection timing is adjusted based on the read results (see Patent Document 1, for example.)
Patent Document 1 discloses an image forming apparatus which includes a pattern forming unit that forms, on a water-repellent member having water-repellent properties, a reference pattern including multiple independent liquid droplets and a pattern to be measured that includes multiple independent liquid droplets ejected under an ejection condition different from the reference pattern such that they are arranged in a scanning direction of a recording head; a reading unit including a light emitting unit which irradiates a light onto the respective patterns and a light receiving unit which receives a regular reflected light from the respective patterns; and a correction unit which measures a distance between the respective patterns based on read results of the reading unit for correcting of a liquid droplet ejection timing of the recording head based on the measurement results.
FIG. 1A is an example of a diagram which schematically describes a light receiving element which reads test patterns. When a spotlight which is irradiated by an LED scans the test pattern in an arrowed direction, a reflected light in accordance with a density of a scanning position of the spotlight is detected at the light receiving element. As is well known, light is absorbed well by a black object, so that it is difficult for the spotlight to be absorbed when the test pattern is scanned if a sheet material is white and the test pattern is black. If the reflected light received by the light receiving element is shown in voltage, as shown, a voltage when the spotlight is superposed on the test pattern is substantially lower than a voltage when what is other than the test pattern is scanned.
FIG. 1B is an example of a diagram showing voltage changes in an enlarged manner. The horizontal axis is time, or a scanning position of the spotlight. An elongated circle shows regions in which the voltage is sharply changing. It is inferred that an edge of the test pattern is within the region, so it is determined, for example, that a center of gravity of the spotlight scans the edge of the test pattern when the voltage value shows a center value of a local maximum and a local minimum. Therefore, when the voltage value shows the median of voltage amplitudes, for example, the image forming apparatus may determine that there is the edge position of the test pattern at the scanning position and specify a position of the test pattern.
However, there is a problem that, when a sheet material is a material with a low reflectance (or a high transmittance) such as a tracing paper, it is difficult for an output voltage of the light receiving element to be stable, so that the edge position of the test pattern may not be specified accurately. In other words, for the sheet material with the low reflectance, a decrease in the amplitude of the voltage value, a variation in the transmittance of the sheet material, or an amplification of sensor sensitivity or transmittance fluctuations of the sheet material causes the voltage value to be unstable. When the amplitude of the output voltage of the light receiving element decreases or becomes unstable, specific accuracy of the edge position of the test pattern decreases, so that accuracy of adjusting liquid droplet ejection timing decreases.
While changing a process of correcting an ejection timing in accordance with a type of sheet of paper may be considered, as the type of sheet of paper is set by a user operation, a case may occur that an adjustment operation suitable for the type of sheet is not possible, leading to a problem that desired adjustment accuracy is not obtained. Moreover, a problem may occur that down time of the image forming apparatus increases when a correction process is performed regardless of the type of paper.